/Engine/Source/Runtime/InteractiveToolsFramework/Public/BaseGizmos/CombinedTransformGizmo.h
Bydefault, non-uniform scaling handles appear (assuming they exist in the gizmo to begin with), when CurrentCoordinateSystem == EToolContextCoordinateSystem::Local, since components can only be locally scaled. However, this can be changed to a custom check here, perhaps to hide them in extra conditions or to always show them (if the gizmo is not scaling a component).
virtual void SetIsNonUniformScaleAllowedFunction ( [TUniqueFunction](API\Runtime\Core\Templates\TUniqueFunction)< bool()> && IsNonUniformScaleAllowed )